Background and Summary of N-methyl Carbamate Revised Cumulative Risk Assessment
Food Quality Protection Act of 1996 (FQPA) directs EPA to consider available information on the cumulative effects on human health resulting from exposure to multiple pesticide chemicals that have a common mechanism of toxicity. This page describes the process used to conduct the N-methyl carbamate cumulative risk assessment.
- Identifying a Common Mechanism Group
- Identifying a Cumulative Assessment Group
- Identifying Exposure Scenarios for the Cumulative Risk Assessment
- Assessment of Cumulative Risk
- The N-methyl Carbamate Revised Cumulative Risk Assessment - more about the September 2007 Federal Register notice and how to comment.
Identifying a Common Mechanism Group
EPA begins a cumulative risk assessment by identifying a group of pesticides, called a common mechanism group, that bring about the same toxic effect by a common mechanism of toxicity. Pesticides share a common mechanism of toxicity if they act the same way in the body; that is, if the same toxic effect occurs in the same organ or tissue by essentially the same sequence of major biochemical events.
The N-methyl carbamate (NMC) pesticides -- aldicarb, carbaryl, carbofuran, formetanate hydrochloride, methiocarb, methomyl, oxamyl, pirimicarb, propoxur, and thiodicarb -- have been identified by EPA as a common mechanism group. They affect the nervous system by reducing the ability of cholinesterase, an enzyme, to function properly in regulating the neurotransmitter acetylcholine. Acetylcholine helps transfer nerve impulses from a nerve cell to a muscle cell or another nerve cell. If acetylcholine is not properly controlled by cholinesterase, the nerve impulses or neurons remain active longer than they should, over-stimulating the nerves and muscles and causing symptoms such as weakness or paralysis of the muscles.
Other pesticides are also considered “carbamate” pesticides. However, only the N-methyl subclass of pesticides is considered in this cumulative risk assessment because the N-methyl carbamates share the ability to inhibit cholinesterase through a specific pathway. Other carbamate pesticides do not share the same common mechanism.
Cholinesterase inhibition was the primary toxicological effect of regulatory concern to EPA in assessing the NMCs’ food, drinking water, and residential risks.
Identifying a Cumulative Assessment Group
Once a common mechanism group is identified, it is important to determine which chemicals from that group should be included in the quantification of cumulative risk. The group of pesticides which is included in the quantification of cumulative risk -- and consequently incorporated into the cumulative risk assessment -- is called the cumulative assessment group. EPA selects a cumulative assessment group with uses, routes, and pathways of exposure that present sufficient exposure and hazard potential to warrant including them in the cumulative risk assessment.
All 10 NMCs identified in this common mechanism group were included in a cumulative assessment group and considered in the cumulative risk assessment.
Identifying Exposure Scenarios for the Cumulative Risk Assessment
After identifying pesticides in a common mechanism group, EPA develops exposure scenarios for the cumulative assessment group, considering pesticide uses that result in exposures. The Agency is especially interested in areas of high exposure and regions of the U.S. where residues of the pesticides in the group are likely to occur together.
EPA identified three exposure pathways of interest for the NMC pesticides: food, drinking water, and residential/non-occupational buildings and public areas. Each of these pathways was initially evaluated separately, and, in performing this portion of the analysis, EPA determined which of the NMCs were appropriate to include for each given pathway.
Food – In evaluating exposure through food, EPA concluded that a few uses of NMC pesticides on food crops generally play a larger role in the results of the assessment. These include: aldicarb on potato; carbaryl on peach and strawberry; and methomyl on cantaloupe, watermelon, peach, and spinach. Although domestic uses of carbofuran have been found ineligible for reregistration, carbofuran import tolerances for four commodities -- bananas, coffee, rice, and sugarcane -- were also included. Once the mitigation measures for the single chemical actions were included, an evaluation of the total risk from exposure to NMCs in foods indicated that the cumulative exposures to NMCs are not of concern.
Drinking Water – In most of the country, NMC residues in drinking water sources are at levels that are not likely to contribute substantially to the multi-pathway cumulative exposure. Shallow private wells extending through highly permeable soils into shallow, acidic ground water represent what the Agency believes to be the most vulnerable drinking water. EPA imposed mitigation measures to address these risks.
Residential/Non-Occupational Buildings and Public Areas – Applications of NMC pesticides in and around homes, schools, offices, and other public areas may result in potential exposure via the oral (due to hand-to-mouth activity by children), dermal, and inhalation routes. There are three NMC chemicals (methiocarb, carbaryl, propoxur) with currently registered residential uses considered as part of the cumulative risk assessment in the residential/non-occupational exposure pathway assessment. The residential uses considered in this assessment include: the methiocarb snail bait use; the carbaryl lawn and golf course uses; the carbaryl vegetable and ornamental garden use; the carbaryl fruit tree use; the carbaryl pet collar uses; and the propoxur pet collar uses. The results of the residential risk assessment indicate that uses of NMCs in residential settings are not of concern.
Assessment of Cumulative Risk
Finally, EPA analyzes high-end exposures to the pesticides in the group via food, drinking water, and/or residential activities in regions where co-occurrence is most likely, and assesses the resulting cumulative risks. Generally, exposures through the food pathway are the dominant risk of concern for the NMCs. Drinking water and residential exposure pathways are substantially less.
The Agency has undertaken extensive risk mitigation and risk reduction efforts over the last several years for many NMCs through the single chemical aggregate risk assessments and notes that the risk mitigation efforts of the past several years have significantly reduced risk from NMCs in food and drinking water, and from residential uses in the United States. Based in large part on these efforts, the cumulative risks from food, water, and residential exposure to NMCs do not exceed the Agency’s level of concern.
Taking into account these reductions and acknowledging that several key assumptions are designed to minimize the potential for this cumulative assessment to underestimate exposure and risk, the Agency concludes that there is a reasonable certainty that no harm will result from exposure to the NMC pesticides covered by this cumulative risk assessment.
The N-methyl Carbamate Revised Cumulative Risk Assessment
In September 2007, EPA completed and made available for public comment its cumulative human health risk assessment for the N-methyl carbamate class of pesticides. The Federal Register Notice announcing the availability of the revised cumulative risk assessment is available on EPA’s Web site.
Concurrent with completing the N-methyl carbamate revised cumulative risk assessment, EPA has completed risk management decisions for the 10 individual N-methyl carbamates. With this reassessment of the remaining tolerances for N-methyl carbamates, EPA has met the goal established by FQPA to ensure that all pesticides used on food in the U.S. meet the more stringent safety standard. All 9,721 tolerances that required reassessment under FQPA now have been re-evaluated, and related risk management decisions are being implemented. Completing tolerance reassessment is an enormous milestone in U.S. food safety and human health protection.